Cambodia has begun preparations to build a controversial hydropower dam on the Se San River.

The Lower Se San 2 Dam will be built downstream on the river, with an 8-kilometre-long wall and a reservoir that will cover more than 300 square kilometres.

Once the waters rise, the surrounding Srekor village will be submerged.

Local farmer Pa Tou is one of an estimated 10,000 people who will have to be relocated.

He says the dam will deprive them of everything - homes, crops, fruit trees and their livelihoods.

Pa Tou says the relocation site the authorities have offered them is miles from the river, bad for farming and does not include amenities.

The wall of the dam will block entry to the Se San River and to the Sre Pok River, affecting the breeding grounds for long-distance migratory fish that make up 40 percent of fish in the Mekong system.

The Lower Mekong Basin is shared by 65 million people in four countries. There are fears that hydropower dams will cause many among the 15-million-strong population of Cambodia to go hungry, as they eat more freshwater fish per capita than any other in the world.

A study published last year in the US Proceedings of the National Academy of Sciences assessed 27 dams that regional governments plan to build on tributaries of the Mekong.

Dr Eric Baran, one of the authors of that study, says the Lower Se San 2 Dam will be the most detrimental for fish.

"We found that the fish yield loss due to this dam would represent 9.3 per cent of the total fish yield of the Mekong Basin," he said.

"So it's 9.3 per cent of 2.1 million tons - which is a gigantic amount.

"In other words, this expected loss represents around 200,000 tons per year, which is much more than the whole marine sector of Australia."

The Lower Se San 2 Dam will cost around $US800 million dollars and take five years to build.

At least two more dams are planned upstream on the Se San River, along with another two scheduled for the neighbouring Sre Pok River; and one on the Se Kong River.

Government officials have declined to speak with the ABC for this story.